Picture yourself flying from Vancouver, Canada to a business meeting in Chongqing, China beginning from leaving your apartment to the meeting site. Here is a wordy description: Before you leave the building, you need to take the elevator. Stairs are too slow and tedious. After leaving the building, you called the taxi to get to the airport: You don't use public transit because you can't afford to wait for buses and you can't handle baggages in crowded spaces.
You don't take the train because they can't travel across the ocean. You don't go on ships because they are too slow. However, you don't get into a fighter jet because supersonic speed is overkill and fighter jets require expensive maintenance after each flight (I think). Commercial airlines are fast and efficient enough, and it has been optimized over and over again to carry many people at a time.
Before you board the plane, you go through the pipelines of checkout, security, baggages, waiting. Wait, I forgot to mention that there's no direct flight from Vancouver to Chongqing, instead, you first fly to Beijing then take another flight to Chongqing.
After you landed in Chongqing, you are still 50km away from the meeting site: Another taxi ride to the front door of the building, and then another elevator ride to the exact floor of the meeting room.
Now here's a more concise, but science fictional example that explains transportation hierarchy: Suppose you are living in an interstellar civilization and you want to take a relativistic flight from a Martian colony in the Sol system to a lunar colony in Alpha Centauri:
- You go to the airport and ride a plane to the equator
- Then you take the shuttle to high Mars orbit (space launches are cheaper in the equator due to planet rotation)
- You arrive at a space station, and you ride a bigger ship to escape from Mars and head to the interstellar shipyard between the orbit of Jupiter and Saturn (It takes months to years)
- You board the relativistic spacecraft, and depart to Alpha Centauri. In the initial acceleration phase, the ship gets accelerated by a megastructure that emit lasers
Now let's think about faster-than-light travel in science fiction: Ever heard of a FTL drive (that's not teleportation or wormholes) that allows you to fly from your house to your friend's house in a distant colony light years away?
The theory of transportation hierarchy is what 22C urban planners subscribe to. It is first developed by a Russian author in late 21C in the book "Travel Fast, Travel Slow: The Theory Behind Getting from Point A to Point B" (erroneously referred as "[The Theory] of Transportation Hierarchy"). Transportation hierarchy is based on the common sense that one does not ride a fighter jet to get to school. Transportation hierarchy is applicable to scales from small villages to interstellar civilizations. Transportation hierarchy is about the traveling over a short or long distance while trading off between energy and time, just like memory hierarchy
is about storing and retrieving a little or a lot of data while trading off between cost and speed.
A transportation hierarchy is a system of ways to get from point A to point B. The hierarchy center around three concepts: distance (or speed), organization and means of transportation. On the top of the hierarchy, larger distances are covered, on the top of geographical organization, and the means of transportation are high-speed and tend to travel in straight lines without stopping. If Earth is the top of the transportation hierarchy, then spacecrafts and long-range aircrafts are the means of transportation that dominate this level. As we move down the hierarchy (state, province, city), the distance covered decrease (50Mm, 1Mm, 100km), the speed decrease (500km/h+, 150km/h, 80km/h). When we reach sub-city levels, we start to deal with moving with cars, public transit or on foot, and the tradeoffs between point-to-point (cars) and spoke-hub (public transit).
Here is a summary of levels of transportation hierarchy, real and science fictional:
Galactic: 10000ly, FTL travel, most paths need straight lines
Interstellar: 100ly, relativistic travel, it's important to start far from the gravity well
Solar system: 10AU, transfer orbits, it's important to carry less mass, time launches well and start higher from planetary gravity wells
Planetary orbit: 1Gm, transfer orbits, it's more useful to measure gravitational potential difference or delta V than distances
International flights: 10Mm
Domestic flights: 5Mm, some common international paths are be point-to-point, while most possible international parts require transfer to hubs
Provincial: 1Mm, Flights or highways depending on the traveller's needs
City, district: 50km (commuting), public transit should cover more possible paths, and for cars, congestion waste time and fuel, cars should approximate the straight line from A to B
Neighborhood: 3km (walking), everything should be within walking distance, but unlike cars, stopping and turning are not burdens
Building: 80m, elevators for tall buildings
An effect of having transportation hierarchy is that the time to get from point A to point B can potentially depend on how many levels traversed, not the distance of the shortest path from A to B. Another way to see it is that negligible distances (for example, the 30km drive to the airport for a 1000km flight) during a trip might not translate to negligible times (the 30km drive and the check-in will take 2 hours, compared to the 2-4-hour flight).
The tradeoffs between point-to-point and spoke-hub, on top of the need to save the environment, are the most visible applications of transportation hierarchy. 22C urban planning involve balancing these factors by artificially constructing levels of transportation hierarchy:
- Speed (walking vs. riding vehicles)
- Energy efficiency
- Scalability and throughput (transportation for a family vs. transportation for a city)
- Flexibility (Point-to-point vs. hubs)
The means of transportation that dominate city-scale are trains, cars and feet, but by changing the structure of roads they travel on, their parameters will change radically.
To make cars travel faster (save time) and stop less often (save energy), 22C urban planners managed to design roads to make cars travel at highway speeds in almost-straight lines more often. Three-dimensional road layouts are what 22C cities can have. The tradeoff is that cars stop at large parking lots below blocks (22C definition: an area of at most 20-30min walking distance) and people need to walk to the precise destination.
To pack more people per square meter, 22C cities are also built to be tall. Skyscraper apartments are common. The elevator is the vertical counterpart of the car, since an elevator, like a car, covers a long distance in short time. The gain for adding elevators is limited. The first optimization is to let passengers enter the destination before stepping into the elevator and the elevator software intelligently schedule the floor to stop at. However, the most radical change 22C architects came up with is the so-called "serial-parallel configurations". In serial-parallel configurations, more elevators are added without increasing the floor areas elevators occupy. The conceptually simplest way to achieve that is to add more independent elevators per shaft. However, the way to solve the problem with transportation hierarchy is to artificially introduce levels of transportation hierarchy.
Let's say there is a 300-floor skyscraper. We can divide the 300 floors into ten 30-floor floor groups. For each floor group, the lowest floors in the group are the master floors for each floor group. There are a few central high-capacity elevators (master elevators) that only stop in the master floors (the vertical counterpart of public transit). For floors within the floor groups, there are elevators that span only within the floor group. Variations include more floor group subdivisions. (The optimal number of floor group subdivisions and what should be in each subdivision is a computational problem)
See also: gizmodo.com/5905096/the-postal…